Method of production of stannous fluoride



Oct, 11, 1960 .1 E. GILLILAND ETAL' 2,955,914

' METHOD OF PRODUCTION 0F sTANNoUs FLUORIDE Filed Aug. Y:26. 1957 H2 f .6m/Me nited States Patent METHOD OF PRODUCTION OF STANNOUS FLUORIDE Joe E. Gilliland, Raymond Ray, and Wayne E. White, Tulsa, Okla., assignors to Ozark-Mahoning Company, Tulsa, Okla., a corporation of Delaware Filed Aug. 26, 1951, ser. No. 680,185

s claims. (cl. zs-ss) plication Serial No. 610,890, led September 20, 1956,V

entitled Method of Producing Stannous Fluoride, now

Patent No. 2,924,508.

Stannous fluoride is presently of great commercial interest as an additive for dentifrices to supply fluorine values to human teeth for the purpose of reducing the Vincidence of dental caries.

The only method for making stannous uoride given inthe literature involves the dissolution of stannous oxide in aqueous hydrofluoric acid. 'This reaction has been de- Yveloped so aspto be able to obtain therefromxhigh purity v lstannots fluoride of the character required to investigate .its character in minimizing dental caries incidence. However, in this reaction, conditions of concentration, ,temperatureand other factors have proved to be of theutmost importance in obtaining a complete interaction of the two reactants. It has also been found quite easy to'miss the ideal conditions and, in such case, the stannous oxide would not be completely dissolved in the acid. Even under the best conditions in this reaction, it is *usually 'necessary to clarify the lsolution in order to Yobtain from it a white, pure salt of stannous iluoride.

In addition to the apparent criticality inl conditions for the reaction, other disadvantages have been encountered in the dissolution of stannous oxide in hydrouoric acid. Filtration of the solutions frequently is difcult and slow. The cost of tin in the form of stannous oxide isrrelatively high. For example, we have recently priced stannous oxide at $1.84 per'pound for the tin in comparison to a t-in metal price of $1.00 per pound. Ad-

ditionally, the available commercial sources of stannous oxide are severely limited as compared tothe ready `sources of metallic tin.

stannous uoride. Tin, being above hydrogen in -the electromotive series of the metals, theoretically should be soluble in acids to liberate hydrogen and to leave a salt of the metal and the anion of the acid. In view of this, we considered the possibility of making stannous fluoride by dissolving metallic tin in hydrotluoric acid. Literature research, however, clearly indicated that such a method was impractical. Not only are all of the methods described in the literature for the production of stannous uoride specific in designating the use of stannous oxide, but, also, many statements in standard literature references indicate the futility of attempting to prepare stannous uoride direcly from metallic tin.

In our patent application Serial No. 610,890, above,

' we were able to demonstrate the error in the literature 1both' in regard to the reactivity of substantially 'anhydrous' Patented Oct. 1l, 1960 fe l Viuoric acid in measured quantities to produce the stannousuoride. Experience proved that while the latter method was operable commercially, it would be desirable for a number of reasons to have an operative method of adding 4the acid to the tin instead of vice versa. The problems of control of the reaction temperature below the melting point of tin, however, were far more severe.

Therefore, an object of the invention is to provide a methodA of reacting metallic tin and hydroiluoric acid which permits the. production of stannous iluoride upon a 'commercial scale and of high purity.

n n Another object of the invention is to provide a method of producing stannous fluoride by the addition of hydrouoric acid to a mass of r'lely divided metallic tin.

Another object of the invention is to'provide a method of reacting metallic tin and hydrouoric acid to produce stannous fluoride by reacting the metallic tin in an excess 'ofl acid, wherein, although the acid is added to the tin,

agglomeration of the metallic tin and fusing thereof is completely avoided.

Another object of the invention is to provide a method of reacting metallic tin in an excess of-hydrofluoric acid tov produce stannous uoride wherein the separation of f excess acid fromthe reaction product is easily and simply accomplished and the separation of the stannous iuoride as crystals from the acidnis also easily accomplishedl Y Another object of the inventionis to provide a method of reacting metallic tin inan excess of hydrofluoric acid in a iirst reaction step to produce stannous iuoride where- Vin the excess acid is recycled to a second reaction step vm'th further tin, a portion of the stannous iluoride produced in the original reaction employed as a' solution to aid -in the separation of newly produced stannous'uoride from the second reaction step. 'i

Yet another object of the invention is to provide va method of producing stannous fluoride by the reaction of metallic tin and hyd-rofluoric acid, wherein the close control of the temperatures of the tin and the reaction lstep permits considerable variance in other variables of the reaction, thereby reducing the criticality of the reaction, yet not sacrificing the purity ofthe product.

Another object of the invention is to provide a method vof controlling critical temperatures in the reaction of hydrouoric acid and metallic tin to produce stannous uoride whereby all of the tin may be reacted in an excess 'of acid and local fusing and agglomeration of the tin due -to excess heating Vin thev reaction step is` completely -lication and is to `be read-in conjunction. therewith, "an embodiment of the invention is shown in 'the form of Va schematic ilow diagramof a portion of the inventive process. f

Referring to the-schematic flow diagram, at 10 is -shown a reaction vessel having input ttings 11 and 12 'which may be'sealed, heat exchanging jacket 13'with input andoutput connections 14and=15 and bottom 'discharge fitting 16. Redux condenser 17 has heat exchange medium input andv output'ttngs. 18and 19. Flow line 20 carries hydrogen gasv and hydrouoricacid vapors to the redux' condenser and Yvvithdravval lini 21 Y-.111 si. ;u1 temperature. 'the lreactor first and cooled, the hydrogen lluoride `does Anot react rapidly when it rst strikes the tin and, before the reactor..

" Ihe process of the reaction of the tinwth.. the acid in. vessel will now be described. The sealontting 121s removed and a charge of relatively ly divided Ymetallic tin is poured into the reactor. vThe, Sealf is. Uilen replaced and the ftin is cooled by circulating coldrbrine or other low temperature heat exchanging medium through the jacket 113 which Ysurrounds. the lower Part of the reactor. With the reactor Yand thectinQled 0 at least a temperature below the boilingfpointof the hydrouoric acid at the pressures of thefreaetion step Yand preferably te about C., eoiueentrat,edY hydrogen iuride isadded as Yrapidly as it will drainfrorn the cylinder. The reaction initially is rather slow but, as the Vtemperature comes up to the boiling pointof the hydro- Aigen-1luoride, the velocityof the reaction inleaseabut never-to an undesirable or uncontrollable. egjrtent.` No .trouble is encountered from hydrogen evolution PFQducling aV froth which is carried out of thereactor.. Redux f condenser -1-7 cools the evolved vapors (1f-hydrogen .and acid and returns the condensed hydrogen fluoride to the reactor via receiving tank 24. The hydrogen is passed through a scrubbing tower to remove any trace of hydrogen fluoride escaping the redux condenser 17.

The interactionof the tin and hydrogen iluorde with Athe production of hydrogen will produce adequatermixingso a mechanical mixer is not required. However, if it is desired to speed up the completion of the reaction,

such mechanical mixing will aid, as Vthe reactionotherwise A'slows' down a good deal after an hour or two. A typical .timek schedule for aY charge of 250 poundsr of powdered tin and 400 pounds of anhydrous hydrogen fluoride, with- Yout mechanical mixing, requires a reaction time of about .critical within a certain'range.

four hours. after bringing the two reactantsy together.

jAfter thisY time, the hydrogen uoride may be vaporized out as will be deselbed later- Y rIt. should.Y be pointed out that a most critical taeter in Y,the reaction is to maintain at least substantially the entire .reaetionstepata temperature below the melting point of the vtiu- The tin itself must be relatively dnely divided :andrnustnot be permitted toaeeumulate enough heat in the exothermic reaction to bring the mass up to its When the tin powder is .placed in ,Sllcient'heat is produced by the reaction to causev this trouble, the volume of hydrogen fluoride in the-reactor is vsuflicient to serve as a coolant. Heat of reaction is dissipated through evaporation of the hydrogen fluoride. To preventexc'essive vaporizationlcss of hydrogen uoride the reactor is equipped with the cold reux condenser.

YThe addition of the hydrogen uoride preferably takes place within a matter'of 15 minutes and may be added partly from a tank retaining the residual amount .from thepreceding run andY partly fresh acid. Once .the hydrogen uoride, in the quantity desired, has been added to the reactor, the brine is'drained from the jacket 13 so that the temperature within the vessel comes up to the boilingy point of hydrogen fluoride at the reaction pressure (19.4. C.at atmospheric). vThe total quantity ofy hydrogen iluoride in the reactor at all times, as controlled by :the 'amount of recycle vfrom the reux condenser, must be suicient to maintain the temperature of the reaction f When hydrogen evolution becomes 'slow',.the' near/completion of the reaction is indicated. The reaction may Ybe completed "Within three to four hoursl'depending` on vvarious factors tovbe set forth. "After completion of the dissolution of the tin, steamV may be passed throughthe "iacketglvS-and the excesshydrogen fluoride yapori'zed and `v`Collected in the receiving* tank 24- maintainedfor Vthat Y 4 purpose. Evaporation of the solution is continued until the liquid is at a temperature of about 65C., at which point it retains 8 to 10% jhydrogen uoride which is necessary to maintain uidity which permits a clean discharge from the reactor.Y

The discharged strong.v solution is received in an aquecus stannous fluoride solution, the filtrate from a preceding batch ofv crystals, or in an auxiliary Closed, iaeketed tank of an alloy resistant to bothaqueous and anhydrous hydrogen iuoride from which the remaining hydrogen luoride is vaporized. In'the first-ease' the majority of the stannous lluoridelfrorn the reactor precipitates out in the aqueous solution until the whole 'is heated whereupon the salt is-dissolved. Then-excess hydrogen fluoride in the solution is neutralized withstannous oxide and the solution is ltered and cooled to crystallize out the pure salt.

lf .the reactor is discharged into an auxiliary jacketed ftank the mixture' of stannous fluoride and hydrogen fluoride is heated further'to. driveV olf additional hydrogen fluoride. The solid remaining is then taken up in hot aqueous stannous fluoride solution circulated through this auxiliary tank,rwhich solution may be the filtrate from a ruececliugY batchof Crystals- As an alternative method ofrecovering the` stannous uoride from the excess hydrogen lluoride remaining in thereactor ,after completion of the reaction, Vthe contents of the reactor may be cooled suiliciently to crystallize out the'stannous nuoride. The residual solution then may Y'be dec'anted orbtherwise removed from the crystalline stannous uoride remaining in the reaction vessel and employed, .if desired, for further use in the system by Vaddition of an rappropriate Yquantity of fresh hydrogen uoride, if needed.'` l

We have found the `concentration of the acid to be With the Ytin powder Vinitially at 15 C., and withracid added topgive aV 5 to 1 acid-tin molar ratio (stoichiometric ratio 2 to l), `inra `four hour period, anhydrous acid gave a 100% complete reaction, acid gave 95 completion, and 75% acid gave 54% completion. We have observed slow reaction of tin and acid at concentration of acid as low as 50% but not at. lowerY concentrations. At 90% acid concentration, we have lfound that, in sixteen hours, we can get al1 of the tin dissolved, that is, we have completion. Thus,Y concentrations Yare important determining the I 'ate of `reaction; anhydrous acid gives the highest rate. A acidy concentration of 5.0% appears to `btf'iequired Afor any measurable reaction. c

In our application Ser. No. 610,890, we disclosed the reaction of tin, and substantially anhydrous hydi'ofluoricV acid, preferably in a process of adding finely divided tin to an excess of acid. We have discovered that this latter VProcess can be performed, though not as successfully and quickly, as when employing more concentrated acid, With hydrogen fluoride in a concentration as low as 50%. Adequate reactiom however, is best Vachieved in the addition of tin to acid in the process of Ser. No. 610,890 with the substantially anhydrous acid of over 70% concentration.

We have found the ratio of acid to tin also affects the rate of completionof the reaction. Employing anhydrous hydrolluoric vacid added to cold tin powder, a ratio of 5 mols of acid to l mol of tin gave 100% dissolution of the tin in two hours, 4 mols acid to lmol tin 97% completion in the same period, 3 mols acid Ato 1 of tin 95.5% completion `and 2.5 mols acid to l of tin 63% completion in a like period. The reaction is conductable with slightly over the stoichiometric amountV of acid relative the tin but the optimum ratio is. at least fourmols acid per mol oftin.

We have found the temperatureV at the time of bringing rvthe reactants together Vto be another critical factor. We have found the upper limit for an effective reaction to be the. temperature above which vthe nely divided Vtin Yaz;-

Maschi/i glomerates so as to reduce its surface `area to a low value. Such agglomeration may consist of a slight sintering of the tin to give a somewhat less reactive mass or it may be a complete fusing so that a compact mass of metal is Obtained which dissolves very slowly. When the tin powder is added to a large volume of anhydrous hydrouoric acid, as in our previous application supra, the liquid acts as a coolant and the hydrogen evolved acts as a means of mixing. Under these latter conditions, the metal does not approach its fusion point because the hydrouoric acid remains at or near its boiling point. However, when the -acid is added tothe tin, an initial reaction may take place before enough liquid is present to maintain the tin temperature below its softening point. We have prevented such undesirable overheating by the cooling the metal before adding the acid. When the temperature of the was reduced to 15 C. before addition of the acid (in the ratio of 5 mols yacid to 1 of tin), the tin dissolution was 100% completed in two hours. When the tin was cooled to 0 `rC. before adding the acid, with the same ratio of reactants, the reaction was 94% complete in two hours. When the tin was cooled to 19.4 C. (the boiling point of the acid at atmospheric pressure) at the same ratio of reactants, 83% reaction occurred in two hours. The most important thing is to prevent localized heating from the reaction. In practice, the optimum precooling is to 15 C.

As to the temperature during the reaction, we have :found that the allowable temperature to be attained in the reaction mixture must be governed by the same gener-al requirement as set forth previously, namely, the metal must not be permitted to reach, even at localized spots, the temperature where it will fuse together. The optimum reaction conditions employ anhydrous hydrofluoric acid with the temperature controlled by acid recycle to remain at the boiling point of this liquid at atmospheric pressure. However, we have operated Iat superatmospheric pressures and elevated temperatures below the melting point of the tin with 75% concentration acid and have had reasonably good results.

As to the size of the particles, it is very diiicult to indicate a nonreactive limit to the size of particles. A tin powder which p asses 99% through 325 mesh is preferable because of its high rate of dissolution. A powder which passes 94% through 325 mesh is workable but slower in reaction. The reaction is very slow with -20 mesh tin granules. Clearly, the particle size relates primarily to the desired reaction rate.

The reux condenser 17 preferably should be operative to cool the evolved gases passing thereto from the reactor to a temperature of 15 C. or below, whereby to -adequately inhibit untimely escape of the hydrouoric acid from the reaction chamber although affording to evolved hydrogen a relatively free exit therefrom. The condenser discharge passage into flow line 20 must be of sucient capacity to allow for escape of the gaseous acid as well as the hydrogen evolved from the reaction. The return by gravity of relatively cold liquid hydrouoric lacid from the rellux condenser, when the latter is kept at 15 C. or below, tends to counteract the heat generated in the exothermic reaction which otherwise might proceed with considerable violence if this or some other expedient were not employed for preventing overheating in the reaction vessel. The acid boils at about room temperature (19.4 C.) at atmospheric pressure and the reaction readily proceeds at approximately the slightly higher boiling point of the reacting mass when the conditions described above are maintained. Condensation of vapor-ized acid, together with cooling of escaping hydrogen, extracts from the system primarily-only heat generated by the exothermic reaction.

It is evident to those skilled in the art that under certain conditions hydrogen fluoride is highly corrosive to materials of construction. In addition to the advantages altrated hydrogen fluoride, a great convenience and economy is found in use of this reagent as ordinary steel can be employed for construction of reacting equipment when the HF concentration is above 70% Aluminum may be satisfactorily used also, but it is more sensitive to the presence of water and consequently cannot be used in concentrations of hydrogen fluoride lower than about We have found also that polyethylene, silver and platinum are satisfactory for carrying out this reaction and the observation that these various materials of construction can be used tends to'negative the possibility that the reacting vessel has any cataly-tic effect in the reaction between the tinv and hydrogen uoride, which reaction it has heretofore been thought could not be carried to completion in respect to either the tin or hydrogen uoride.

It is believed our discovery that stannous fluoride is soluble in an excess of concentrated hydrouoric acid at about its boiling point has contributed materially to the success of our method in that by virtue of the solubility reaction of the tin. If SnF2 were insoluble in the acid,

-it would be expected to deposit `at the site of production,

that is, on unreacted tin, and thereby materially retard the dissolution of the remaining metal.

As the crystalline product obtained by the practice of our method is soluble in cold water, it is apparent that it may be added to dentifrices in the manufacture thereof by any suitable or desired procedure in the proportions deemed appropriate for presenting on contact with the teeth of the consumer a proper concentration of a fluorine bearing compound, nontoxic at concentration employed but adequate to reduce the incidence of den-tal caries, about .4% stannous uoride in toothpaste and comparable concentrations in other forms of dentifrices usually being considered adequate and not injurious to the health of the user.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth together with other advantages which are obvious and which are inherent to the structure.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter hereinabove set forth or shown `in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

Having thus described our invention, We claim:

l. A method of producing stannous uoride comprising the steps of first cooling a quantity of relatively nely divided metallic tin to a temperature substantially below the boiling point of hydrouoric acid at the pressures of the reaction step, said tin cooled to a temperature suiciently below the boiling point of liquid hydroiluoric acid at the pressures of the reaction step to substantially prevent any reaction of the tin upon contact with liquid hydrouoric acid, then adding a quantity of liquid hydroiiuoric acid of greater than 50 percent concentration toV said tin in a reaction step, then permitting the temperature of the reaction mixture to rise to the temperature of the boiling point of the solution at the pressures existent in the reaction step whereby to permit the reaction of the tin and the acid, maintaining the reaction step at least substantially in its entirety at a substantially uniform temperature not greater than the boiling point of the liquid present in the reaction step, by maintaining a sufcient excess of liquid hydroiluoric acid continuously present in the reaction step and in contact with at least substantially all of the tin reacting in the reaction step, whereby to prevent at least substantially any agglomeration of the tin into unreactive metallic bodies in said reaction step due to fusion thereof in the presence of excess heat, and

.recovering stannous fluoride from the resultant solution. 2. lA method of producing stannous fluoride by contactngtin and hydrouoric acid in the reaction step comprising the. steps ofirst cooling a quantity ofrelatively finely divided metallicitin to a temperature substantially below the boiling point lof hydroiiuoric acid at the pressures, of the V,reaction step, said tin cooled to a temperature sniciently Vbelow the boiling point of liquidhylrofiuoric acid y, at the pressures ofthe reaction Ystepto substantially prevent any reaction Vof the tin upon contact ofv liquid hydrotiuoric acid therewith, then adding a quantity of liquid hydrouoric acid of greater than 5() percent concentration to said tin, the quantity of hydrouoric acid added being inl'excess ofthe molar quantitiesrepresented by the ,equation:

then permitting the temperature ofthe reaction mixture to rise to they temperaturel ofv the boiling point of thessolution atV the pressures existent in the reaction step whereby `to permit complete reaction of the tin and acid, maintaining a sucient quantity ofhydrouoric acidApresent' at alltimes in the reaction'l step and in contact with atleast substantially all of the tinA reacting in the reaction step, so

as to maintain the reaction step .at least substantially in its entirety at a substantially uniformV temperature not greater than the boiling point of the liquid present in the reaction step, said hydrouoric acid in excess-of the amountof Ahydroiluoric acid required to dissolve the tin, whereby to prevent, at least substantially any agglomeration of the tin into unreactive metallic bodies in said reaction step due Vto fusion thereof in the presence of excess heat, and recovering stannous fluoride from the resultant solution.

3. A method as Vin claim 2 wherein the step Yof recovering stannous fluoride from the resultant solution includes raising the temperature of the reaction mixture aftercompletionl of the reaction above the boiling point of the acidfwhereby to vaporize a quantity thereof and removing said vapor from said reaction step.

V4. A method as in claim 3 including the step of discharging the acid-stannous'fiuoride mixture after removal of said vapor into an aqueous solution of stannous uoride to precipitate stannous fluoride crystals.

5. A method as in claim 3 includ-ing the steps of discharging the acid-stannous uoride mixture after removal of saidvapor into an aqueous solution vof stannous iluoride to precipitate stannous uorde crystals, neutralizing the mixture of the aqueous solution of stannous uoride and acid solution by reaction of the acid with stannous oxide and then cooling same to crystallize out stannous uoride.

References Cited in the iile of this patent (Oct. 1952). 

1. A METHOD OF PRODUCING STANNOUS FLUORIDE COMPRISING THE STEPS OF FIRST COOLING A QUANTITY OF RELATIVELY FINELY DIVIDED METALLIC TIN TO A TEMPERATURE SUBSTANTIALLY BELOW THE BOILING POINT OF HYDROFLUORIC ACID AT THE PRESSURES OF THE REACTION STEP, SAID TIN COOLED TO A TEMPERATURE SUFFICIENTLY BELOW THE BOILING POINT OF LIQUID HYDROFLUORIC ACID AT THE PRESSURES OF THE REACTION STEP TO SUBSTANTIALLY PREVENT ANY REACTION OF THE TIN UPON CONTACT WITH LIQUID HYDROFLUORIC ACID, THEN ADDING A QUANTITY OF LIQUID HYDROFLUORIC ACID OF GREATER THAN 50 PERCENT CONCENTRATION TO SAID TIN IN A REACTION STEP, THEN PERMITTING THE TEMPERATURE OF THE REACTION MIXTURE TO RISE TO THE TEMPERATURE OF THE BOILING POINT OF THE SOLUTION AT THE PRESSURE EXISTENT IN THE REACTION STEP WHEREBY TO PERMIT THE REACTION OF THE TIN AND THE ACID, MAINTAINING THE REACTION STEP AT LEAST SUBSTANTIALLY IN ITS ENTIRETY AT A SUBSTANTIALLY UNIFORM TEMPERATURE NOT GREATER THAN THE BOILING POINT OF THE LIQUID PRESENT IN THE REACTION STEP, BY MAINTAINING A SUFFICIENT EXCESS OF LIQUID HYDROFLUORIC ACID CONTINUOUSLY PRESENT IN THE REACTION STEP AND IN CONTACT WITH AT LEAST SUBSTANTIALLY ALL OF THE TIN REACTION IN THE REACTION STEP, WHEREBY TO PREVENT AT LEAST SUBSTANTIALLY ANY AGGLOMERATION OF THE TIN INTO UNREACTIVE METALLIC BODIES IN SAID REACTION STEP DUE TO FUSION THEREOF IN THE PRESENCE OF EXCESS HEAT, AND RECOVERING STANNOUS FLUORIDE FROM THE RESULTANT SOLUTION. 